I've recently been working on a Flow that required the creation of a number of fairly complex expressions. It can get pretty frustrating trying to do this within the extremely limited confines of the expression editor (not to mention the intellisense helper that keeps jumping up and blocking the Dynamic content link). I think the user experience would be improved if there was a way to open a larger window for the expression editor when you need the extra space for longer expressions (similar to the expression builder in SharePoint Designer).